In U.S. Pat. Nos 4,559,405 and 4,560,773 to J. Telschow, a process for preparing substituted phthalic anhydrides is described. The process involves reacting the Diels-Alder addition product of a conjugated diene and maleic anhydride (e.g., 4-methyl-1,2,3,6-tetrahydrophthalic anhydride) with bromine in the presence of a catalytic amount of an acid acceptor such as dimethylformamide or pyridine. The use of dimethylformamide catalyst (which is termed "most preferred" in those patents) has, however, given rise to less than desired yields and a higher than desired impurity level (e.g., of 4,N-dimethyl phthalimide) resulting in less than desired assay values. It was found, moreover, that the boiling point of the impurity was essentially the same as that of the desired substituted phthalic anhydride thereby precluding its separation or removal by distillation techniques. Hence, a need has arisen for an improved process of the general type illustrated in the Telschow patents which would result in a higher yield of product and a lower level of impurity.